Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 67)

Publication 1790-UM003A-EN-P
Thermocouple Descriptions C-5
The suggested upper temperature limit, 870°C, given in the ASTM
standard [7] for protected type E thermocouples applies to AWG 8 (3.25
mm) wire. It decreases to 650°C for AWG 14 (1.63 mm), 540°C for AWG
20 (0.81 mm), 430°C for AWG 24 or 28 (0.51 mm or 0.33 mm), and 370°C
for AWG 30 (0.25 mm). These temperature limits apply to thermocouples
used in conventional closed-end protecting tubes and they are intended
only as a rough guide to the user. They do not apply to thermocouples
having compacted mineral oxide insulation.
Type J Thermocouples This section discusses Iron Versus Copper-Nickel Alloy (SAMA)
thermocouples, called type J thermocouples. A type J thermocouple is
one of the most common types of industrial thermocouples, because of its
relatively high Seebeck coefficient and low cost. It has been reported that
more than 200 tons of type J materials are supplied annually to industry in
this country. However, this type is least suitable for accurate thermometry
because there are significant nonlinear deviations in the thermoelectric
output of thermocouples obtained from different manufacturers. These
irregular deviations lead to difficulties in obtaining accurate calibrations
based on a limited number of calibration points. The positive
thermoelement is commercially pure (99.5 percent Fe) iron, usually
containing significant impurity levels of carbon, chromium, copper,
manganese, nickel, phosphorus, silicon, and sulfur. Thermocouple wire
represents such a small fraction of the total production of commercial iron
wire that the producers do not control the chemical composition to
maintain constant thermoelectric properties. Instead, instrument
companies and thermocouple fabricators select material most suitable for
the thermocouple usage. The total and specific types of impurities that
occur in commercial iron change with time, location of primary ores, and
methods of smelting. Many unusual lots have been selected in the past,
for example spools of industrial iron wire and even scrapped rails from an
elevated train line. At present, iron wire that most closely fits these tables
has about 0.25 percent manganese and 0.12 percent copper, plus other
minor impurities.
The negative thermoelement for type J thermocouples is a copper-nickel
alloy known ambiguously as constantan. The word constantan has
commonly referred to copper-nickel alloys containing anywhere from 45
to 60 percent copper, plus minor impurities of carbon, cobalt, iron, and
manganese. Constantan for type J thermocouples usually contains about
55 percent copper, 45 percent nickel, and a small but thermoelectrically
significant amount of cobalt, iron, and manganese, about 0.1 percent or
more. It should be emphasized that type JN thermoelements are NOT
generally interchangeable with type TN (or EN) thermoelements, although
they are all referred to as “constantan”. In order to provide some
differentiation in nomenclature, type JN is often referred to as SAMA
constantan.